The invention relates to new polymer materials with non-linear optical properties and processes for their preparation.
Non-linear optics are concerned with the interaction of electromagnetic fields in various media and the associated formation of new fields with changed properties. Materials with non-linear optical properties have a dielectric susceptibility which depends on the field strength and results in a number of dispersive processes: frequency doubling (second harmonic generation=SHG) allows generation of light of half the wavelength compared with the incident light; the electrooptical effect (Pockels effect) allows a change in the refractive index when a direct current electrical field is applied; methods of sum and difference frequency mixing and frequency distribution permit continuous adjustment of laser light.
The abovementioned effects result in a large number of technical applications. Optical switches and waveguides for construction of purely optical computers, frequency and intensity control in laser technology, holography and the sectors of information processing and integrated optics are exemplary fields of use for materials with non-linear optical properties.
In order to be suitable for use in the field of non-linear optics, such materials must fulfill a number of requirements. One such requirement is that arrangement of the molecules in crystalline form should be non-centrosymmetric. In addition, technical usefulness requires that the materials have the maximum possible values for dielectric susceptibility, .chi.(Y. R. Shen, The Principles of Nonlineaar Optics, Chapter VI, John Wiley, New York, 1985).
A number of inorganic substances, such as, for example, potassium dihydrogen phosphate or lithium niobate, exhibit non-linear optical properties. However, all these compounds have diverse disadvantages. As well as inadequate values of the second order dielectric susceptibility, inorganic compounds frequently have the deficiency of inadequate photostability during treatment with high light intensities or, as a result of being highly colored, inadequate transparency.
Organic compounds of the nitroaniline type are known from Garito et al., Laser Focus 18 (1982) and European Pat. No. 0,091,838. Their relatively good values for photochemical stability and second order dielectric susceptibility are accompanied, however, by a poor crystallizability and a lack of mechanical stability. In particular, it is not possible to prepare thin layers, as required by integrated optics, with these materials.
Polymers are distinguished by a high mechanical resistance and good chemical stability. Molecules with non-linear optical properties attached to the polymer skeleton or dissolved in the polymer should therefore have high mechanical resistance and good chemical stability combined with advantageous values of dielectric susceptibility in the non-centrosymmetric environment.
Polymers with second order non-linearities can be prepared by applying an external field to polymer films heated above the glass transition temperature and doped with randomly oriented molecules. This leads to poling of the embedded molecules which imparts anisotropy to the polymer medium after the medium has solidified. Polymers which are prepared in this manner, have non-linear optical properties. Such doped polymer systems wherein p,p'-dimethylamino-nitrostilbene is used as the host molecule have been described by Meredith et al., Macromolecules 15 (1982) 1385.
Shibaev et al., Polymer Communications 24 (1983) 364 report field-induced alignment of liquid crystal polymers with mesogenic side groups.
U.S. Pat. No. 4,412,059 discloses a polymer material with cholesteric mesophases which are accessible by means of electric or magnetic fields with a controlled alignment. In addition, fully aromatic, thermotropic, liquid crystal polymers in which the non-linear optical properties can likewise be caused by external fields are known from European Pat. No. 0,172,012.
Another method for producing polymer materials with non-linear optical properties comprises polymerization of already ordered monomers with a non-centrosymmetric orientation, the order of this system largely being retained during the polymerization. Monomers which are suitable for this technique are to be found, for example, in European Pat. No. 0,021,695.
The materials obtained by the processes described above still have unsatisfactory non-linear optical properties. Only incomplete alignment of the composite polymer is obtained by the action of an external field, which is an additional process step. In the pre-ordered monomers incomplete alignment results from orientation losses occurring during polymerization.